This invention relates to a method for mixing concrete using a cementitious material liquid premixer. More particularly, this invention relates to a method whereby the cementitious material portion and liquid portion of concrete are precisely metered and thoroughly mixed to form a flowable slurry within an enclosed screw conveyor before being mixed together with aggregate (e.g. sand and gravel) in a final product mixing chamber. Because the cementitious material and liquid are fully enclosed during the flowable slurry mixing step, the amount of airborne cementitious particulate matter, i.e. "dust," that is usually attendant in such mixing methods is greatly reduced. The need to apply copious quantities of water to wash the dust off the mixing equipment and other surfaces in the mixing area is also proportionately reduced. Moreover, the time required to mix a given quantity of concrete is likewise reduced using this method, which results in more efficient equipment utilization and greater output.
Increasingly strict local, state and federal pollution regulations have become an onerous burden to the operators of concrete mixing plants, particularly small mixing plant operators. Limits on airborne particulates and groundwater runoff and contamination require expensive modifications to existing concrete mixing plant equipment and operating procedures. New equipment that has become available only incidentally addresses these problems, and is complex and generally unsuitable for an existing mixing plant retrofit.
In addition to the need to reduce airborne particulates and groundwater runoff and contamination, there is an increased awareness that water is a very finite resource that needs to be conserved. While water is a minor component in the concrete mixture per se, it is a major component in the cleanup process for the concrete mixing area.
In addition, there are potential quality control issues that can arise when a specific concrete mixture requires a precise ratio of materials. Materials that are carefully measured should also be added together in a precise metered manner and thoroughly mixed to produce complete hydration. Obviously, when a portion of the cement that has been carefully measured according to a ratio for inclusion in a mixture is lost as airborne particulate, the characteristics of the final concrete mixture are altered. Likewise, mixing equipment that relies primarily on gravity to dispense and meter cement can easily clog, resulting in uneven metering, mixing, and an inferior end product.
Prior art improvements in the field of concrete mixing apparatus have generally been either technically complex attempts to solve particular problems affecting the very specific needs of small segments of the industry, or attempts to increase overall efficiency.
Ono et al. U.S. Pat. No. 5,100,239 discloses a method to produce concrete for mass concrete members by spraying liquid nitrogen onto aggregate (particularly sand) within enclosed conveyor screws prior to combining the nitrogen cooled aggregate with cement, water, and coarser aggregates for the final mixing operation. Ono does not recognize nor address the need to control cement dust pollution in a concrete mixing system by providing an inexpensive retrofittable apparatus.
Raypholtz U.S. Pat. No. 2,486,323 discloses a complicated variable output mixing system for mixing aggregate and bituminous material that operates similar to a pugmill without recognition of the foregoing pollution problem.
Owen U.S. Pat. No. 1,753,716 discloses a screw conveyor mixer particularly suited to producing a grout mixture for cementing oil wells. Owen does not provide nor suggest a final product mixing chamber for mixing a flowable cement slurry with aggregate to form concrete, nor recognize the foregoing pollution problem.
Haws U.S. Pat. No. 4,586,824 discloses a mobile concrete mixing apparatus wherein a conveyor initially carries aggregate from a storage bin. Dry cement is dumped on top of this aggregate as it travels on the conveyor, and water is sprayed on the aggregate and dry cement as it is dumped into a feed screw for mixing. Nothing in the system prevents cement dust pollution.
Dunton et al. U.S. Pat. Nos. 4,904,089 and 4,830,505 disclose a method of mixing particulate cement and water in a primary mixing vessel to form a slurry and delivering the slurry to an auxiliary mixing vessel for mixing with aggregate. The method and apparatus disclosed in Dunton '505 and '089 illustrates the recognized desirability and advantages produced by premixing concrete and liquid to form a slurry before mixing with aggregate. However, Dunton's solution is very complex and expensive, requiring the use of high velocity pumps and multiple rotary agitators to create the flowable slurry, and lacking easy retrofit adaptability to existing concrete mixing plants.